A high-performance secondary cell, such as lithium ion cell, which recently has come into widespread use, is characterized by a small size, lightness in weight and a large capacity, Lithium ion cells are used in increasing numbers in a portable equipment, such as a portable telephone set or a note-type computer.
However, this type of the high-performance secondary cell has a high energy density in the cell, While a combustible organic solvent is occasionally used as an electrolytic solution, such that, in case of an emergency, such as during shorting or overcharging in the cell, combustible gases tend to be evolved or ignited in the cell vessel and raise the internal pressure in the cell thus producing a problem such as inadvertent leakage of the cell contents--therefor;
If the internal pressure in the vessel exceeds the compressive strength, the vessel is deformed excessively. If the limit of compression is exceeded, the vessel explodes.
If such accident occurs during use of the portable equipment, the user is endangered or the equipment is destroyed.
For avoiding this risk, a cleavage type safety valve which, if the internal pressure in the vessel is increased beyond a prescribed value, part of the vessel is cleaved to form an opening to open the internal pressure.
The cleavage type safety vale, used up to now, is classified into a marking type safety valve and a metal foil type safety valve, as selected depending on usage and application.
The marking type safety valve is of such a structure in which a marker punch having a tooth end with a wedge-shaped cross-section is driven into a portion of a vessel for machining a cleavage groove to a circular or the like contour so that, if the internal pressure of the vessel is increased to higher than a prescribed value, the thin-walled portion of the bottom of the cleavage groove is cleft to open the internal pressure.
The marker type safety valve has merits in facilitated machining and low manufacturing cost.
However, in driving the marker punch, cracks generated at the tooth end of the marker punch are left in the thin-walled portion after machining, such that, if the thin-walled portion is of a reduced wall thickness, cracks are formed extending through the thin-walled portion, thus possibly producing leakage.
The cleavage pressure in the marker type safety valve is determined by the pressure-receiving surface surrounded by the cleavage groove and the plate thickness of the thin-walled portion of the bottom of the cleavage groove. Thus, if it is desired to reduce the pressure-receiving surface for installment in a small-sized vessel and nevertheless it is desired to prevent the cleavage pressure from increasing, the plate thickness of the thin-walled portion needs to be decreased further. However, if the plate thickness of the thin-walled portion is further reduced, the ratio of rejection due to leakage is increased. Thus, difficulties are met in reducing the size of the marker type safety valve.
The metal foil type safety valve is of such a structure in which a metal foil is attached for closing the air vent hole in the vessel for hermetic sealing, so that, if the internal pressure in the vessel exceeds a prescribed value in case of an emergency, the metal foil becomes cleft to open the internal pressure.
In the conventional metal foil type safety valve, the metal foil is attached by a welding method. Since the lower limit value of the plate thickness of the metal foil that can be attached by the welding method is not more than one-half the lower limit value of the plate thickness of the thin-walled portion of the bottom of the cleavage bottom in case of the marking type cleavage cell, the metal foil type safety valve can, in this respect, be reduced more easily in size.
However, in attachment of the metal foil by the welding method, there are encountered problems of through-holes proper to the welding processing and technical difficulties in attachment and hermetic sealing by the welding method. In addition, equipment investment becomes costly thus presenting cost and quality problems.